Question: The cubic polynomial $p(x)$ satisfies $p(2) = 1,$ $p(7) = 19,$ $p(15) = 11,$ and $p(20) = 29.$  Find
\[p(1) + p(2) + p(3) + \dots + p(21).\]
Answer: The cubic passes through the points $(2,1),$ $(7,19),$ $(15,11),$ and $(20,29).$  When these points are plotted, we find that they form the vertices of a parallelogram, whose center is $(11,15).$  We take advantage of this as follows.

[asy]
unitsize(0.2 cm);

real func (real x) {
  real y = 23*x^3/585 - 253*x^2/195 + 7396*x/585 - 757/39;
	
	return(y);
}

pair A, B, C, D;

A = (2,1);
B = (7,19);
C = (15,11);
D = (20,29);

draw(graph(func,1.5,20.5),red);
draw(A--B--D--C--cycle,dashed);

label("$(11,15)$", (11,15), NE, UnFill);
dot("$(2,1)$", A, SW);
dot("$(7,19)$", B, W);
dot("$(15,11)$", C, SE);
dot("$(20,29)$", D, NE);
dot((11,15));
[/asy]

Let $f(x) = p(x + 11) - 15.$  Then
\begin{align*}
f(-9) &= p(2) - 15 = -14, \\
f(-4) &= p(7) - 15 = 4, \\
f(4) &= p(15) - 15 = -4, \\
f(9) &= p(20) - 15 = 14.
\end{align*}Now, let $g(x) = -f(-x).$  Then
\begin{align*}
g(-9) &= -f(9) = -14, \\
g(-4) &= -f(4) = 4, \\
g(4) &= -f(-4) = -4, \\
g(9) &= -f(-9) = 14.
\end{align*}Both $f(x)$ and $g(x)$ are cubic polynomials, and they agree at four different values, so by the Identity Theorem, they are the same polynomial.  In other words,
\[-f(-x) = f(x).\]Then
\[15 - p(11 - x) = p(x + 11) - 15,\]so
\[p(11 - x) + p(x + 11) = 30\]for all $x.$

Let
\[S = p(1) + p(2) + p(3) + \dots + p(21).\]Then
\[S = p(21) + p(20) + p(19) + \dots + p(1),\]so
\[2S = [p(1) + p(21)] + [p(2) + p(20)] + [p(3) + p(19)] + \dots + [p(21) + p(1)].\]Since $p(11 - x) + p(x + 11) = 30,$ each of these summands is equal to 30.  Therefore,
\[2S = 21 \cdot 30 = 630,\]and $S = 630/2 = \boxed{315}.$